1. Field of the Invention
The present invention relates generally to steel body rotary drag bits and, more specifically, to retention of generally cylindrical cutting elements within steel bodied rotary drag bits for drilling subterranean formations.
2. State of the Art
Steel bodied rotary drag bits employing cylindrical polycrystalline diamond compact (“PDC”) cutters have been employed for drilling subterranean formations for a relatively long time. PDC cutters comprised of a diamond table formed under ultra-high temperature, ultra-high pressure conditions onto a substrate, typically of cemented tungsten carbide (WC), were introduced about twenty-five years ago. Steel drill bit bodies are typically fabricated by machining a piece of steel to form generally radially extending blades, cutting element sockets or pockets, junk slots, internal watercourses and passages for delivery of drilling fluid to the bit face, ridges, lands, and other external topographic features of the drag bit. A threaded pin connection for securing the drill bit body to the drive shaft of a downhole motor or directly to drill collars at the distal end of a drill string rotated at the surface by a rotary table or top drive may typically be machined separately from a different steel grade and then may be affixed to the bit body by welding.
Conventional cutting element retention systems generally comprise two styles: (1) tungsten carbide studs comprising a cylindrical tungsten carbide cylinder having a face oriented at an angle (backrake angle) with respect to the longitudinal axis of the cylinder, the face carrying a superabrasive cutting structure thereon, wherein the cylinder is press-fit into a recess that is generally oriented perpendicularly to the blades extending from the bit body on the bit face; and (2) mechanical and/or brazed attachment of a generally cylindrical cutting element into a recess formed on the bit face, typically on a blade extending therefrom. Regarding the first cutting element retention style, PDC cutting elements may be brazed to the face, or other superabrasive structures may be affixed thereto, by infiltration or brazing, such as thermally stable diamonds (TSPs). Accordingly, the first cutting element retention style is designed for a stud-type cutting element, while the second cutting element retention style is designed for generally cylindrical cutting elements, such as PDC cutters. In either system, the goals are to provide sufficient cutting element attachment and retention as well as mechanical strength sufficient to withstand the forces experienced during the drilling operation.
Of the two different types of cutting element retention configurations utilized in the manufacture of steel body rotary drill bits, generally cylindrical cutting elements are generally preferred and almost uniformly utilized therefor. Stud-type cutting elements, on the other hand, are relatively uncommon and may require a brazing or infiltration cycle to affix the PDC or TSPs to the stud. Therefore, it may be preferable to form a recess into a steel body bit blade that has the shape of a flat-ended, right cylinder. Often, the preferred method of machining a flat-ended cylinder is by plunging a rotating flat-bottomed machining tool, such as an end mill disposed at the angle desired for backrake into the rotationally leading face of a bit blade along the axis of rotation of the end mill. Such a machining operation may yield a cutting element pocket having a substantially cylindrical surface and a substantially planar end surface for disposing and brazing a generally cylindrical cutting element therein.
Although generally cylindrical cutting elements are almost uniformly employed in manufacturing steel body rotary drill bits, difficulties may arise in machining the recesses therefor within the steel body. For instance, there may be interference between the machining equipment used, such as a multiple-axis milling machine, and the drill bit blades. More specifically, the interference may inhibit a desired machining path of a machining tool that is aligned generally along the axis of rotation thereof because the collet or chuck that retains the machining tool may contact an adjacent blade.
Notwithstanding use of a right angle converter to reduce the amount of clearance required, or a longer machining tool which may allow for the collet or chuck holding the machining tool to be positioned at a greater distance from the bit body, in steel-body rotary drill bit designs where adjacent blades are relatively close to one another, interference may still exist. Therefore, bit designs including blades that are relatively near to each other may prevent effective machining of cutting element pockets because an adjacent bit blade may intersect the projection of the cutting element recess geometry itself. Put another way, in order to form the desired cutting element recess having an arcuate surface for conforming to the generally cylindrical portion of a generally cylindrical cutting element and a substantially planar end surface for supporting the generally cylindrical cutting element by way of a flat-bottomed machining tool, such as an end mill, the machining tool may be required to remove a portion of the rotationally leading adjacent blade. As a further complication, drill bit profile designs often taper longitudinally away from the direction of drilling precession as the profile approaches the center of the face of the drill bit. Thus, near the center of the bit, use of a flat-bottomed machining tool to form recesses for generally cylindrical cutting elements within steel body rotary drill bits may be extremely difficult. For this reason, steel body rotary drill bit design may be limited in flexibility in order to utilize the relatively popular generally cylindrical cutting element.
As shown in FIGS. 1A and 1B, conventional steel body rotary drill bit body 10 may typically comprise generally longitudinally extending and radially-directed upwardly projecting blades 34. Cutting element pockets 30 may be formed within blades 34 proximate intervening junk slots 36 for retaining cutting elements (not shown) for engaging and cutting the formations during rotation of the conventional steel body rotary drill bit body 10 as known in the art. In addition, nozzle cavities 18 may be formed for accepting nozzles (not shown) for communicating drilling fluid from the interior of the steel body rotary drill bit body 10 to the cutting elements (not shown) and face 38 of the conventional steel body rotary drill bit body 10. As known in the art, conventional steel body rotary drill bit body 10 may be affixed to a bit shank to form a steel body rotary drill bit wherein the shank includes an end for connection to a drill string or, alternatively, to a down hole drill motor assembly.
Cutting element pockets 30 formed in blades 34 are of a general right cylindrical shape as shown in FIGS. 1A and 1B. As may be further seen with respect to FIGS. 1A and 1B, cutting element pockets 30 proximate the inner radial region 26 of conventional steel body rotary drill bit body 10 may be difficult to form conventionally. Further, as blades 34 extend nearer to one another, especially within the inner radial region 26 of conventional steel body rotary drill bit body 10, conventional cutting element pockets 30 may become difficult to form. As may be further noted, a cutting element pocket 30 of a conventional steel body rotary drill bit body 10 may not fully support the substantially planar surface of a cutting element disposed therein because the cutting element pocket 30 may extend only to the top surface of the blade. However, the conventional cutting element pockets 30 may be machined in such a way as to form supporting backings (not shown) that extend above the upper surface of the blade 34, in conformity with the substantially planar surface of a generally cylindrical cutting element (not shown), but such machining may be time intensive and expensive.
Furthermore, generally cylindrical cutting elements (not shown) may typically be brazed within the cutting element pockets 30 formed within the conventional steel body rotary drill bit body 10. While brazing may be generally adequate under moderate drilling conditions, generally cylindrical cutting elements may fracture during drilling, and conventional brazing configurations may not prevent the fractured portion of the generally cylindrical cutting elements from becoming detached from the conventional steel body rotary drill bit body 10, and may thereby likely cause damage to other generally cylindrical cutting elements affixed thereto.
U.S. Pat. No. 4,453,605 to Short discloses a metallurgical and mechanical holding of cutters in a matrix-type rotary drill bit.
U.S. Pat. No. 5,056,382 to Clench discloses a method for forming the displacements within a mold to form matrix cutter pockets by way of two independent end mill passes within a matrix-type rotary drag bit mold.
U.S. Pat. No. 5,558,170 to Thigpen et al. discloses a cylindrical cutting element having a spherical end that may be mechanically locked by the side walls of the recess formed therefor.
Therefore, it would be advantageous to provide an improved cutting element retention configuration for use in steel body rotary drag bits. Further, it would be advantageous to provide a cutting element retention apparatus that is implementable by way of conventional machining equipment and improves flexibility of design. In addition it would be advantageous to provide a cutting element retention apparatus that provides mechanical locking of at least a portion of the cutting element within the steel body rotary drill bit.